The electronic display is an indispensable way in modern society to deliver information and is utilized in television sets, computer terminals and in a host of other applications. No other medium offers its speed, versatility and interactivity. Known display technologies include plasma displays, light emitting diodes (LEDs), thin film electroluminescent displays, and so forth.
The primary non-emissive technology makes use of the electro optic properties of a class of organic molecules known as liquid crystals (LCs) or liquid crystal displays (LCDs). LCDs operate fairly reliably but have relatively low contrast and resolution, and require high power backlighting. Active matrix displays employ an array of transistors, each capable of activating a single LC pixel. There is no doubt that the technology concerning flat panel displays is of a significant concern and progress is continuously being made. See an article entitled “Flat Panel Displays”, Scientific American, March 1993, pgs. 90–97 by S. W. Depp and W. E. Howard. In that article, it is indicated that by 1995 flat panel displays alone are expected to form a market of between 4 and 5 billion dollars. Desirable factors for any display technology is the ability to provide a high resolution full color display at good light level and at competitive pricing.
Color displays operate with the three primary colors red (R), green (G) and blue (B). There has been considerable progress in demonstrating red, green and blue light emitting devices (LEDs) using organic thin film materials. These thin film materials are deposited under high vacuum conditions. Such techniques have been developed in numerous places throughout the world and this technology is being worked on in many research facilities.
Presently, the most favored high efficiency organic emissive structure is referred to as the double heterostructure LED which is shown in FIG. 1A and designated as prior art. This structure is very similar to conventional, inorganic LED's using materials as GaAs or InP.
In the device shown in FIG. 1A, a support layer of glass 10 is coated by a thin layer of Indium Tin Oxide (ITO) 11, where layers 10 and 11 form the substrate. Next, a thin (100–500 Å) organic, predominantly hole transporting layer (HTL) 12 is deposited on the ITO layer 11. Deposited on the surface of HTL layer 12 is a thin (typically, 50 Å–100 Å) emission layer (EL) 13. If the layers are too thin there may be lack of continuity in the film, and thicker films tend to have a high internal resistance requiring higher power operation. Emissive layer (EL) 13 provides the recombination site for electrons injected from a 100–500 Å thick electron transporting layer 14 (ETL) with holes from the HTL layer 12. The ETL material is characterized by its considerably higher electron than hole mobility. Examples of prior art ETL, EL and HTL materials are disclosed in U.S. Pat. No. 5,294,870 entitled “Organic Electroluminescent MultiColor Image Display Device”, issued on Mar. 15, 1994 to Tang et al.
Often, the EL layer 13 is doped with a highly fluorescent dye to tune color and increase the electroluminescent efficiency of the LED. The device as shown in FIG. 1A is completed by depositing metal contacts 15, 16 and top electrode 17. Contacts 15 and 16 are typically fabricated from indium or Ti/Pt/Au. Electrode 17 is often a dual layer structure consisting of an alloy such as Mg/Ag 17′ directly contacting the organic ETL layer 14, and a thick, high work function metal layer 17″ such as gold (Au) or silver (Ag) on the Mg/Ag. The thick metal 17″ is opaque. When proper bias voltage is applied between top electrode 17 and contacts 15 and 16, light emission occurs through the glass substrate 10. An LED device of FIG. 1A typically has luminescent external quantum efficiencies of from 0.05 percent to 4 percent depending on the color of emission and its structure.
Another known organic emissive structure referred as a single heterostructure is shown in FIG. 1B and designated as prior art. The difference in this structure relative to that of FIG. 1A, is that the EL layer 13 serves also as an ETL layer, eliminating the ETL layer 14 of FIG. 1A. However, the device of FIG. 1B, for efficient operation, must incorporate an EL layer 13 having good electron transport capability, otherwise a separate ETL layer 14 must be included as shown for the device of FIG. 1A.
Presently, the highest efficiencies have been observed in green LED's. Furthermore, drive voltages of 3 to 10 volts have been achieved. These early and very promising demonstrations have used amorphous or highly polycrystalline organic layers. These structures undoubtedly limit the charge carrier mobility across the film which, in turn, limits current and increases drive voltage. Migration and growth of crystallites arising from the polycrystalline state is a pronounced failure mode of such devices. Electrode contact degradation is also a pronounced failure mechanism.
Yet another known LED device is shown in FIG. 1C, illustrating a typical cross sectional view of a single layer (polymer) LED. As shown, the device includes a glass support layer 1, coated by a thin ITO layer 3, for forming the base substrate. A thin organic layer 5 of spin-coated polymer, for example, is formed over ITO layer 3, and provides all of the functions of the HTL, ETL, and EL layers of the previously described devices. A metal electrode layer 6 is formed over organic layer 5. The metal is typically Mg, Ca, or other conventionally used metals.
An example of a multicolor electroluminescent image display device employing organic compounds for light emitting pixels is disclosed in Tang et al., U.S. Pat. No. 5,294,870. This patent discloses a plurality of light emitting pixels which contain an organic medium for emitting blue light in blue-emitting subpixel regions. Fluorescent media are laterally spaced from the blue-emitting subpixel region. The fluorescent media absorb light emitted by the organic medium and emit red and green light in different subpixel regions. The use of materials doped with fluorescent dyes to emit green or red on absorption of blue light from the blue subpixel region is less efficient than direct formation via green or red LED's. The reason is that the efficiency will be the product of (quantum efficiency for EL)*(quantum efficiency for fluorescence)*(1-transmittance). Thus a drawback of this display is that different laterally spaced subpixel regions are required for each color emitted.